1. Field of the Invention
The present invention relates to a method of producing an inert gas mixture. More specifically, the present invention relates to rendering a high-oxygen gas mixture inert to prevent the occurrence of fire if exposed to an ignition source. This feature is particularly valuable when applied to medical applications, such as during surgery or home health care, where high oxygen levels are necessary during medical procedures or health-impaired conditions.
2. Related Art
Surgery room fires, which occur when breathable high-oxygen blends administered to patients come into contact with local combustible materials and tissues, and ignition sources such as laser tools and cauterizers, result in patient injuries or fatalities that are not that uncommon. This serious situation has been recently discussed in several national television documentary programs, as attested by the following quote from the “OSHA Compliance” website (www.osha-compliance.com):                “Recently, the popular news program “60 Minutes” ignited the fears of the American public by demonstrating how easily fires can occur during routine surgery. Of the several hundred surgical fires that occur each year, more than a few result in serious harm to the sedated patient.”        
The rationale for the high risk of fire in this situation is that patients require large relative concentrations of oxygen to breathe, sometimes 30-55% by volume or greater, when their constitution is weak during surgery or with other debilitating pulmonary illnesses, and such a high oxygen concentration makes a myriad of materials easy to ignite, and very difficult to prevent from igniting. Fires can often occur within the windpipe and lungs of a patient, due to laser tools or during cauterization while performing a routine tracheotomy or similar procedures. When this occurs, surgeons often instinctively move back from the flash fire, then after gathering their bearings, attempt to quench the fire by closing the wound from the outside to try to starve it. Unfortunately, the oxygen is already present within the burning region, and by the time the hazard is brought under control, severe pulmonary or other regional burn injuries have occurred, which either result in fatalities, or painful, debilitating injuries. It is also common that oxygen can leak underneath the breathing mask, and accumulate around operating table curtains and other combustible textile materials, which exposed to a high oxygen source can facilitate ease of ignition under circumstances where ignition could not occur in normal atmospheric conditions.
This hazard is also present where oxygen tanks are used in a home health care environment, with approximately 4 million individuals using such equipment in the U.S. In this circumstance, other ignition sources are commonly present, such as cigarettes and other electrical equipment, and numerous residential fire department calls respond to such catastrophic fire events. For these applications, high oxygen concentrations may also be employed, comparable to operating room applications; however the likelihood of ignition and fire may be even higher in this circumstance due to less control over the combustibles exposed in the environment, as well as minimal training of users as to the proper procedures for use of such oxygen equipment.
It is extremely difficult to identify classes of gaseous compounds that can be added to high-concentration oxygen mixtures and accomplish inertion against ignition when the mixture is applied to significant ignition sources and combustible materials, at inertant concentrations that still retain a sufficient oxygen concentration needed for most of these applications in normal use. It is even more difficult to identify classes of compounds that are successful inertants under such restrictions, and yet are sufficiently safe to breathe in the required inerting concentrations for humans during normal use, including individuals with less than ideal health.
Carhart et al (U.S. Pat. No. 3,893,514) discloses a means of suppressing fires in confined spaces, by sealing the chamber and discharging super-pressurized nitrogen into the chamber, at a sufficient quantity to extinguish fires, at a final chamber pressure of 1.5 to 2 atmospheres. This approach exploits the principle by which humans and other mammals require a minimal partial pressure of oxygen of 0.2 to 0.3 atmospheres to normally function, regardless of the local pressure, while fires require a minimum oxygen relative atmospheric concentration of 10-15% to be sustained. By discharging nitrogen to generate excess atmospheric pressure, the relative oxygen concentration can drop below this level to prevent the sustainment of fires, yet retain the minimal partial pressure of oxygen to sustain human occupation. No mention is made of effectively extinguishing or preventing fires at elevated oxygen levels required for medical applications, and some surgical operations require access to the airways of the patient that would preclude containment and sustainment of the volume at high pressures.
Dougherty et al discloses (U.S. Pat. No. 5,040,609) the use of compositions containing CHF3 as a means of preventing or extinguishing fires. It can be used in compartments in volume percentages as high as 80% in a safe manner for human exposure, at least as claimed at the time by the applicants. It is disclosed solely for use in compartments, and not administered directly to humans. It is also disclosed as for use in normal atmospheric air conditions, and not in elevated oxygen compositions. In fact, in calculations disclosed in the application, it is seen that the lower effectiveness of the chemical results in the requirement of 62% concentration in air to prevent or suppress fires, with only 21% of oxygen present, with minimal further expansion of the permitted oxygen concentration, as the required chemical concentration would then require even greater amounts.
Robin et al discloses (U.S. Pat. No. 5,117,917) a range of perfluorinated products, of the chemical family CXF2X+2, that are disclosed to address pre-existing fires. This family of chemicals, and in particular C4F10, are disclosed as being efficient in extinguishing files, while also exhibiting no ozone depletion and low toxicity. It was not disclosed whether the chemical(s) could be applied to high oxygen environments, or directly to a human subject.
Huggett discloses (U.S. Pat. No. 3,715,438) a means of using perfluorocarbon chemicals to prevent and control fires, by mixing them in appropriate quantities with air, in a manner to create habitable atmospheres and sustain mammalian life. To determine the correct amount of inertant required to prevent combustion, Huggett established that atmospheric mixtures that exhibit a threshold heat capacity of 40-50 cal/C (dependent upon the flammability of the present combustibles) per mole of oxygen present have too much heat absorptive capability to permit a fire to be sustained. To produce an atmospheric blend of these specifications, Huggett devised a formula to determine the quantities of various inertants that could raise the bulk heat capacity to that level (with greater quantities reducing the oxygen level). He found that the high heat capacity perfluorocarbons were an ideal candidate for consideration, exhibiting high heat capacities, low boiling points and very low toxicity. The scope of Huggett's analysis was producing a non-combustible atmosphere at an oxygen level comparable to that normally encountered on Earth—i.e., 19-21% oxygen blended atmospheres. He analyzed exclusively atmospheres beginning with normal air, with dilution due to the addition of the perfluorocarbon inertant, and in some cases the addition of some “make-up” oxygen to restore the blend to near-normal terrestrial conditions of oxygen concentration. At oxygen concentrations of that level, as little as 5.2 to 12.6% of perfluoropropane was necessary (corresponding to threshold heat capacities of 40 to 50 cal/C, respectively) to inert atmospheres with final oxygen concentrations of 21% (the same as normal air). Huggett defined the “habitable” conditions of his scope as those permitting “normal” activities for occupants. The “makeup” oxygen, when employed, was to replenish oxygen respirated in the closed, hermetic system, as opposed to a steady feed system. In the discussion of safe oxygen partial pressures, it was stated that is was understood (at the time) that a range from 1.8 psia to 8.2 psia were the limits of safe exposure, which was mentioned in context since at the time in the Space Program, such a maximum total pressure of pure oxygen was used in the atmosphere of the Lunar Modules at the time. It was mentioned in the context of preferring an “ideal” oxygen level of 18-21% for Huggett's embodiment, for “normal” activities of the scope of his invention. In summary, the specification and claims of Huggett reveal several characteristics—(1) they are all focused on modified “air”-based atmospheres that feature a significant component of nitrogen; (2) the focus of the specifications and totality of the claims focuses on maintaining merely “sufficient” oxygen to sustain mammalian life, which by his definition provides an atmosphere to permit occupants to conduct “normal” activities for extended periods, elaborated in the text as being preferentially 18-21%, (3) only perfluorocarbons and nitrogen are mentioned as inertant components under consideration, (4) any “make-up” oxygen is supplied to sustain this “sufficient” amount that is otherwise lost due to respiration or other losses, (5) discharge is expected to be a single event, with “make-up” oxygen only applied as needed to restore steady conditions in the enclosure, as opposed to a steady flow of blended atmosphere to the subject, and (6) all the claims and discussion refer to deployment of this atmosphere within an entire enclosure, versus being administered directly to a human being.
Huggett also discloses (U.S. Pat. No. 3,840,667) a follow-on application that elaborates further on this approach. In it, it discloses adding helium as the primary component within the specification, and as an essential component of every claim, purportedly to add a low-molecular weight additive to the other high-molecular weight inertant to bring the overall average molecular weight to that more closely approaching that of regular air, presumably for increased comfort of those exposed to it. A range of oxygen partial pressures was disclosed to facilitate the invention's use in a wide variety of atmospheric pressures, from sub-atmospheric on space platforms, to above atmospheric in hyperbaric chambers, but at an overall oxygen concentration within the “optimal” limits for comfort for normal activities.
In summary, it is desired to provide a system and special-purpose, artificial composition of oxygen and other compounds (rather than air-based derivatives), including various components other than just perfluorocarbons, that render the composition inert and unable to support ignition and flame when exposed to ignition sources and combustible materials, with oxygen volumetric concentrations in the composition of at least 30-55% or more, which is needed for medicinal and recuperative processes that extend beyond “normal” activities, and the levels of oxygen concentration required to perform them. Additionally, such a composition and system must be designed so as to also support human (or mammalian) life when it is administered directly to an individual, typically in a medical application, in a manner providing steady flow to the subject, although other applications such as manned space platforms, submarines and other enclosed spaces are possible. No technique has been demonstrated that incorporates these features previously, in their entirety.